To overcome some types of hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers by way of one or more channels formed by an array of electrodes implanted in the cochlea. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
When a cochlear implant system is initially implanted in a patient, and during follow-up tests and checkups thereafter, it is usually necessary to fit the cochlear implant system to the patient. Such “fitting” includes adjustment of the base amplitude or intensity of the various stimuli generated by the cochlear implant system from the factory settings (or default values) to values that are most effective and comfortable for the patient. For example, the intensity or amplitude and/or duration of the individual stimulation pulses provided by the cochlear implant system may be mapped to an appropriate dynamic audio range so that the appropriate “loudness” of sensed audio signals is perceived. That is, loud sounds should be sensed by the patient at a level that is perceived as loud, but not painfully loud. Soft sounds should similarly be sensed by the patient at a level that is soft, but not so soft that the sounds are not perceived at all.
Hence, fitting and adjusting the intensity of the stimuli and other parameters of a cochlear implant system to meet a particular patient's needs requires the determination of one or more most comfortable current levels (“M levels”). An M level refers to a stimulation current level applied by a cochlear implant system at which the patient is most comfortable. M levels typically vary from patient to patient and from channel to channel in a multichannel cochlear implant.
M levels are typically determined based on subjective feedback provided by cochlear implant patients. For example, a clinician may present various stimuli to a patient and then analyze subjective feedback provided by the patient as to how the stimuli were perceived. Such subjective feedback typically takes the form of either verbal (adult) or non-verbal (child) feedback. Unfortunately, relying on subjective feedback in this manner is difficult, particularly for those patients who may have never heard sound before and/or who have never heard electrically-generated “sound.” For young children, the problem is exacerbated by a short attention span, as well as difficulty in understanding instructions and concepts, such as high and low pitch, softer and louder, same and different. Moreover, many patients, such as infants and those with multiple disabilities, are completely unable to provide subjective feedback.
Hence, it is often desirable to employ an objective method of determining M levels for a cochlear implant patient. One such objective method involves applying electrical stimulation with a cochlear implant system to a patient until a stapedius reflex (i.e., an involuntary muscle contraction that occurs in the middle ear in response to acoustic and/or electrical stimulation) is elicited. This is because the current level required to elicit a stapedius reflex within a patient (referred to herein as a “stapedius reflex threshold”) is highly correlated with (e.g., in many cases, substantially equal to) an M level corresponding to the patient. However, currently available techniques for measuring the current level at which a stapedius reflex actually occurs within a cochlear implant patient are unreliable, time consuming, and difficult to implement (especially with pediatric patients).
For example, a middle ear analyzer is often used to objectively measure a sound level at which an acoustic stimulus elicits a stapedius reflex in a non-cochlear implant patient by applying the acoustic stimulus to the ear of the non-cochlear implant patient and recording the resulting change in acoustic immittance. It would be desirable for a middle ear analyzer to be adapted for a cochlear implant patient by configuring the middle ear analyzer to record a change in acoustic immittance that occurs in response to electrical stimulation provided by the cochlear implant system. The change in the acoustic immittance could then be used to derive the stapedius reflex threshold.
However, it is currently difficult and time consuming for a clinician to use separate and unsynchronized devices to apply electrical stimulation and measure the resulting change in acoustic immittance. For example, the clinician may direct the cochlear implant system to step through a plurality of current levels as the middle ear analyzer records the resulting change in acoustic immittance. However, because the middle ear analyzer is not synchronized with the cochlear implant system (i.e., the middle ear analyzer does not “know” which current level is being applied by the cochlear implant system at any given time), it is impossible for the middle ear analyzer to correlate the recorded changes in acoustic immittance with the various current levels that are applied to the patient. Hence, the acoustic immittance recordings generated by the middle ear analyzer may be difficult or even impossible to interpret.